1. Field of the Invention
The present invention relates to the field of plasma processing and, more particularly, to the use of plasma equipment for modification of materials.
2. Prior Art
Plasma processing equipment is used extensively in the industry for the process modification of materials. These modifications include etching and depositing of films for the fabrication of microelectronic circuits and semiconductor devices. The modifications may also include implantation of chemical species that change the friction and wear properties of surfaces.
A plasma is a gas (or a gas mixture) which is energized so that it is partially decomposed into species that are electrically charged. Although a number of techniques are known for energizing the gas, electron cyclotron resonance (ECR) plasma processing is one known technique for controlling the plasma with the use of electric (E) and magnetic (B) fields.
ECR plasma production involves the interaction of microwave power with a magnetized plasma at a particular resonant frequency. A well-known and popularly used ECR practice is based on resonance values given by the cold plasma theory. The magnetic field for this resonance is proportional to the microwave frequency and has a magnetic value of 875 Gauss (G) at the resonant microwave frequency of 2.45 gigahertz (GHz). The prior art abounds with references pertaining to the production of plasma at the 875 G/2.45 GHz ECR resonance. Two descriptive articles on ECR plasma production are 1) "Electron cyclotron resonance microwave discharges for etching and thin-film deposition", Jes Asmussen; J.Vac. Sci. Technol. A, Vol. 7, No. 3; May/June 1989; pages 883-893; and 2) "Microwave plasma: its characteristics and applications in thin film technology"; J. Musil; Vacuum, Vol. 36, Nos. 1-3; 1986; pages 161-169.
The plasma technology has been utilized in the semiconductor industry for the purpose of processing the surface of a semiconductor substrate, such as a silicon wafer. Typically, the plasma is used to deposit a layer onto a wafer or, alternatively, it is used to etch a layer on the wafer. A variety of plasma reactors are known in the prior art and the semiconductor industry has developed many enhancements to these reactor designs.
For example, U.S. Pat. No. 4,101,411 discloses an apparatus wherein the surface of a substrate is etched by using ions in a generated plasma. U.S. Pat. No. 4,298,419 describes an apparatus which has a plasma generating chamber for the generation of the plasma and a separate etching chamber where the substrate sample is processed. Subsequent adaptation of the ECR technology was then utilized to practice the technology described in 3) "CVD utilizing ECR plasma"; Transactions of 31st Semiconductor Integrated Circuit Technology Symposium; Dec. 3-4, 1986; pages 49-54; which is shown as prior art in FIG. 1 of U.S. Pat. No. 4,876,983.
However, it soon became apparent that the long transport distance from the plasma formation region to the specimen resulted in the loss (sometimes substantial loss) of the plasma energy, tending to result in the density of the plasma reaching the specimen to be lesser than that of the plasma at or near the resonance zone. Furthermore, the divergence of the magnetic flux lines in the radial direction tended to cause the plasma to diverge as it is transported to the wafer, also resulting in reduced plasma density at the wafer relative to the resonance zone. This technique appears to work reasonably well for processes which are not ion dominated (that is, high plasma densities are not required at the wafer), which do not need low ion energies, and/or which do not require rapid processing.
Although the earlier reactor technology provided desired results for semiconductor wafer processing in the past when wafer diameters were in the order of 4-5 inches, these prior art techniques appear to be inadequate for the processing of larger diameter wafers. As silicon semiconductor technology advances to manufacture devices on wafers having submicron dimensions of 0.5 micron and below, such as 0.35, 0.25, 0.15 micron and below, the industry has moved toward single wafer processing utilizing larger diameter wafers. It has been shown experimentally that as wafer size increases, at and beyond diameters of 200 mm (8 inch), and where topographic features on the wafer are made ever smaller, uniformity is a critical constraint in performing ECR processing.
A variety of techniques have been tried to attempt to provide a more uniform processing over the complete surface of the wafer. One technique is described in U.S. Pat. No. 4,876,983 in which the formation of the plasma (in essence the location of the resonance) is moved closer to the specimen. This is achieved by the ECR zone being located at least partially within the specimen chamber and nearer to the wafer.
A related technique is described, in an article entitled 4) "Extremely high selective, highly anisotropic, and high rate electron cyclotron resonance plasma etching for n+ poly-Si at the electron cyclotron resonance position"; Samukawa et al; J. Vac. Sci. Technol. B, Vol. 8, No. 6, Nov./Dec. 1990; pages 1192-1198; which suggested that a source which would allow processing at higher pressures (over 2 to 3 mTorr) within a few (2 to 10) cm of the ECR zone would reduce density gradients and thus ion energies. In order to preserve uniformity, the stated criterion was that the ECR zone of 875 gauss (G) should have flat contour in space. That is, the suggested technique was to have a flat resonance zone in the reactor and the divergence of the field was to be reduced to maintain the highest possible density with minimum ion energy. The divergent magnetic field is corrected and collimated by the use of a submagnetic field.
However, having a substantially flat magnetic field contour in itself does not assure the formation of a plasma having uniform density across the complete surface of the wafer. That is, for example, uniform microwave power density must be transmitted to the location of the resonance for the formation of a plasma having uniform density.
Experimentations have shown that some plasma control can be obtained by the use of multipole surface magnetic fields to confine the ECR discharge. U.S. Pat. Nos. 4,483,737 and 4,745,337 describe the use of this technique. A multipole design for maintaining a low magnetic field and good uniformity is disclosed in U.S. Pat. No. 5,032,202.
Some additional control is possible by using microwave couplers with different radial profiles of microwave power which is incident on the microwave window as determined by the waveguide mode of the microwave coupler. An example of such an approach is the use of the TM.sub.01 mode instead of the more typical TE.sub.01 rectangular or TE.sub.11 circular modes. However only limited control is possible as the laws of physics constrain the number of possible modes for the dimensions and frequencies of typical processing systems.
Although fairly uniform ECR discharges can be obtained with the above prior art technology, uniform plasma discharge to the wafer is difficult to achieve especially for submicron device fabrication on larger diameter wafers.